Monoterpene Glycosides, State-of-the-Art

Before 1969, when Francis and Allcock reported the occurrence of geranyl, neryl, and citronellyl /I-D-glucosides in rose petals, nothing was known about such non-steam volatile forms of monoterpenoids in plant tissue, although the increase of essential oil content during storage and similar phenomena involved the existence of such compounds. To date, 25 different plants have been investigated, and 22 different monoterpene glycosides, mainly glucosides, have been detected. Geraniol, nerol, linalol and a-terpineol represent the monoterpenoid aglycones most often detected, all four being key intermediates in monoterpene biosynthesis. This fact may indicate a close connection between the monoterpene glycosides and the monoterpene biosynthesis. anthorpe and co-workers consider monoterpene glucosides as structural equivalents to the carbonium ions in Ruzicka's hypothetical scheme of monoterpene biosynthesis, while Francis postulates geranyl glucoside to be an intermediate in the step from geranyl pyrophosphate to geraniol. Other results show the glycosides as a transport form of the free monoterpenes, on the one hand from the site of biosynthesis to the site of accumulation, on the other hand in the service of monoterpene turnover and catabolism. On account of their hydrophilic properties their role as a transport form seems plausible and opens a wide field for future research work.